High-workability steel pipe and method of producing same

ABSTRACT

A steel pipe is produced by a method including performing diameter-reducing rolling on a steel pipe in a temperature range of from 600° C. to Ac 3  with a reduction in diameter of not less than 30%, preferably after heating the steel pipe to temperatures of not lower than Ac 1,  the steel pipe being produced by seam-welding strip steel, or a method further including the step of performing heat treatment of holding the rolled steel pipe in a temperature range of from 600° C. to 900° C. for a time of 1 second or longer during cooling subsequent to the diameter-reducing rolling or by reheating the rolled steel pipe after the cooling.

TECHNICAL FIELD

This disclosure relates to a steel pipe having superior workability anda method of producing the steel pipe.

BACKGROUND

For the purpose of reducing the weight and cost, the application of seam(electric resistance) welded steel pipes to automobile parts has beenconsidered. Conventional seam welded steel pipes, however, have not beensufficient in workability. Bending is employed to manufacture, e.g.,undercarriage or suspension parts of automobiles. When the conventionalseam welded steel pipes are subjected to the bending, a problem has beenexperienced in that a pipe wall is greatly thinned on the outer side ofa bent portion, and in the worst case a pipe is ruptured. Even in thecase of not causing a rupture, a large rate of thinning of the pipe wallrequires the use of a material having a greater thickness to satisfy thedesign stress, and therefore a sufficient reduction in weight cannot beachieved.

As disclosed in Japanese Unexamined Patent Application Publication No.55-56624, for example, it is known that improving an r-value (Lankfordvalue) of a pipe in the axial direction is effective to overcome theproblems described above. As a method for increasing the r-value of asteel pipe, however, it is only known to increase the r-value of stripsteel as a base material of a steel pipe as disclosed in, for example,Japanese Unexamined Patent Application Publication No. 6-41689. Whenproducing seam welded steel pipes, there has been a problem that ther-value is reduced in a portion where melting or transformation of asteel material has occurred during seam welding. Another problem hasarisen in that the seam welding cannot be applied to steel plates nothaving a high r-value, such as hot-rolled steel plates, high tensilestrength steel plates, and low, medium and high carbon steel plates.

Accordingly, it could be advantageous to provide a steel pipe beingsuperior in workability, particularly in bending workability, in whichan r-value of the pipe in the axial direction in a portion where meltingor transformation of a steel material has occurred during seam weldingis as high as comparable to that in a portion where melting ortransformation of the steel material has not occurred, and a method ofproducing the steel pipe.

SUMMARY

With the view of overcoming the problems mentioned above, we conductedstudies based on a consideration that working and heat treatment of seamwelded steel pipes are required to improve the r-value in a weldedportion near the seam. Then, we studied a method of performing workingand heat treatment of a steel pipe evenly at any positions in thecircumferential direction, the steel pipe being produced by seam-weldingcold-rolled steel having a high r-value. We found that the r-value ofthe seam welded steel pipe in the longitudinal direction (in the axialdirection of the pipe) is noticeably improved to 1.2 or above, inparticular to 1.6 or above, at any positions in the circumferentialdirection, including a seamed portion, by a method of performingdiameter-reducing rolling on the seam welded steel pipe in a temperaturerange of from 600° C. to Ac₃ with a reduction in diameter of not lessthan 30% (referred to as “the method” or “our method” hereinafter).

As a result of applying the method to seam welded steel pipes producedusing various kinds of steel plates as base-material strip steel, wealso found that a high r-value can be obtained regardless of the r-valueof the original strip steel. Further, we found that with the method, therestriction of ingredients which has hitherto been employed to obtain ahigh r-value in steel sheets, i.e., a reduction of the C and N contentsand addition of stabilizing elements such as Ti and Nb, are notrequired. As a result, seam welded steel pipes having a high r-value canalso be produced using, as base-material strip steel, hot-rolled steel,high tensile strength steel such as dual phase steel, and low, mediumand high carbon steel, which have a difficulty in achieving a highr-value in the stage of strip steel.

Our views regarding the reason why a steel pipe having a high r-valuecan be obtained from even a steel plate not having a high r-value are asfollows.

By performing the diameter-reducing rolling on a seam welded steel pipein a temperature range of from 600° C. to Ac₃ with a reduction indiameter of not less than 30%, an ideal aggregation structure due to therolling, in which the <110> axis is parallel to the longitudinaldirection and the <111 > to <110> axes are parallel to the radialdirection, is formed and then further developed through restoration andrecrystallization. That aggregation structure provides a high r-value.The aggregation structure due to the rolling produces very great drivingforces because crystals are rotated by working strains. Unlike anaggregation structure that is created through recrystallization in thecase of obtaining a high r-value in steel sheets, the aggregationstructure due to the rolling is less affected by the second phase andsolid solution C. Consequently, even for the type of strip steel whichhas a difficulty in obtaining a high r-value in the stage of producingsteel plates, a high r-value can be obtained in the stage of producingsteel pipes.

Also, the reason why a high r-value is not obtained by performing thediameter-reducing rolling at low temperatures is that ideal crystalrotation is not caused because of high work hardness, or thatrestoration and recrystallization are not developed at a sufficientlevel because of low temperatures. Furthermore, the reason why a highr-value is not obtained by a method of performing the diameter-reducingrolling on a steel pipe at low temperatures and then annealing therolled steel pipe for recrystallization is that the desired aggregationstructure is not developed through the cold rolling and therecrystallization because of the effect of the second phase and solidsolution C.

In the field of producing steel sheets, there is known a method ofproducing a steel sheet having a high r-value by rolling steel into asheet in the hot ferrite range. That method of producing a steel sheethaving a high r-value is featured in that steel containing C and N inreduced amounts and added with stabilizing elements such as Ti and Nb isrolled at low temperatures and then recrystallized. That sheet rollingat low temperatures differs from the diameter-reducing rolling at hightemperatures intended by our method. In fact, if the known sheet rollingin the hot ferrite range is carried out at 600° C. or above, the r-valueis not improved, but rather noticeably lowered on the contrary. This isbecause, in the sheet rolling in which draft is applied in the thicknessdirection of a sheet, strain occurs in a direction different from thatin the diameter-reducing rolling of a steel pipe in which draft isapplied in the circumferential direction, and hence the aggregationstructure effective in increasing the r-value is not developed.

As a result of further continuing the studies, we found that, in ourmethod, the thickness deviation can be noticeably reduced and theoccurrence of wrinkles near the seam can be suppressed by heating a seamwelded steel pipe to temperatures of not lower than Ac₁ before thediameter-reducing rolling for austenitic transformation of a part or thewhole of a steel structure, because the difference in mechanicalproperties between the hardened structure of the seam and the remainingportion is reduced. We therefore provide:

(1) A high-workability steel pipe wherein an r-value in the longitudinaldirection is not less than 1.2, more preferably not less than 1.6, overan entire area in the circumferential direction, including a seamedportion.

(2) A method of producing a high-workability steel pipe, the methodcomprising the step of performing diameter-reducing rolling on a steelpipe in a temperature range of from 600° C. to Ac₃ with a reduction indiameter of not less than 30%, the steel pipe being produced byseam-welding strip steel.

(3) A method of producing a high-workability steel pipe, the methodcomprising the steps of heating a steel pipe to temperatures of notlower than Ac₁, the steel pipe being produced by seam-welding stripsteel, and then immediately or after cooling and reheating the steelpipe, performing diameter-reducing rolling in a temperature range offrom 600° C. to Ac₃ with a reduction in diameter of not less than 30%.

(4) In the method of producing a high-workability steel pipe defined inthe above (2) or (3), after the diameter-reducing rolling of the steelpipe, heat treatment of holding the rolled steel pipe in a temperaturerange of from 600° C. to 900° C. for a time of 1 second or longer isperformed during cooling subsequent to the diameter-reducing rolling orby reheating the rolled steel pipe after the cooling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between an r-value in thelongitudinal direction of a steel pipe having been subjected todiameter-reducing rolling and a reduction in diameter.

FIG. 2 is a graph showing the relationship between an r-value in thelongitudinal direction of a steel pipe having been subjected todiameter-reducing rolling and an outgoing-side temperature in therolling process.

FIG. 3 is a graph showing the relationship between a seam thicknessdeviation in a steel pipe having been subjected to diameter-reducingrolling and a heating temperature before the diameter-reducing rolling.

DETAILED DESCRIPTION

In our high-workability steel pipe, an r-value in the longitudinaldirection is not less than 1.2. The reason is that the bendingworkability of the steel pipe is noticeably improved when the r-value isnot less than 1.2. More preferably, the high-workability steel pipe hasan r-value of not less than 1.6 because the bending workability isfurther improved when the r-value is not less than 1.6.

The high-workability steel pipe can be produced by performingdiameter-reducing rolling on a steel pipe in a temperature range of from600° C. to Ac₃ with a reduction in diameter of not less than 30%, thesteel pipe being produced by seam-welding strip steel and having a seam.The r-value is affected by the reduction in diameter and the temperatureduring the diameter-reducing rolling.

FIG. 1 is a graph showing the relationship between the r-value in thelongitudinal direction and the reduction in diameter at circumferentialpositions 0°, 90°, 180° and 270° of each steel pipe which was producedby performing the diameter-reducing rolling on a seam welded steel pipeunder a condition of the outgoing-side temperature being set to 730° C.while changing the reduction in diameter of the seam welded steel pipefrom that produced by an ordinary method from strip steel having thesame composition as steel A in Table 1 given below. The seam position isassumed to be at 0° (this is similarly applied to the followingdescription). From FIG. 1, it is understood that, regardless of thecircumferential positions, the r-value of not less than 1.3 is obtainedat the reduction in diameter of not less than 30%, and the r-value ofnot less than 1.6 is obtained at the reduction in diameter of not lessthan 50%.

FIG. 2 is a graph showing the relationship between the r-value in thelongitudinal direction and the outgoing-side temperature resulted atcircumferential positions 0°, 90°, 180° and 270° of each steel pipewhich was produced by performing the diameter-reducing rolling on a seamwelded steel pipe under a condition of the reduction in diameter set to30% while changing the outgoing-side temperature, the seam welded steelpipe being produced by an ordinary method from strip steel having thesame composition as steel A in Table 1 given below. From FIG. 2, it isunderstood that the r-value of not less than 1.2 is obtained at theoutgoing-side temperature of not lower than 600° C.

Based on the experiment results mentioned above, a lower limit of thetemperature for the diameter-reducing rolling was set to 600° C. and alower limit of the reduction in diameter was set to 30%. Also, an upperlimit of the temperature for the diameter-reducing rolling was set tothe same as an upper limit of the temperature range in which the steelstructure contains ferrite, i.e., the temperature Ac₃. The r-value isnot improved even by the diameter-reducing rolling if it is performed onsteel whose structure contains no ferrite. The temperature Ac₃ dependson the chemical composition of steel, and can be determined based onexperiments. A range of temperature Ac₃ is approximately not higher than900° C. So long as the steel structure contains ferrite, the secondphase (phase other than ferrite) is not limited to particular one. Forexample, austenite may be the second phase. More preferably, thediameter-reducing rolling is performed at temperatures where ferriteforms the main phase (phase having a volume ratio of 50% or more).

We subject a steel pipe to the diameter-reducing rolling in atemperature range where the steel structure has the ferrite phase. Fromthe standpoint of improving the r-value, there is no particularrestriction upon the history prior to the diameter-reducing rolling. Forexample, the heating temperature prior to the diameter-reducing rollingmay be any of the temperature at which the steel structure has thesingle austenitic phase, the temperature at which the steel structurehas the two austenitic and ferrite phases, and the temperature at whichthe steel structure has the single ferrite phase. Further, prior to thediameter-reducing rolling, the steel pipe may be rolled at suchtemperatures as forming austenite as the single phase or the main phase.

FIG. 3 is a graph showing the relationship between a heating temperatureand a thickness deviation resulted for each steel pipe which wasproduced by performing the diameter-reducing rolling on a seam weldedsteel pipe under conditions of the reduction in diameter set to 30% andthe rolling temperature set to 700° C. while changing the heatingtemperature, the seam welded steel pipe being produced by an ordinarymethod from strip steel having the same composition as steel A in Table1 given below. From FIG. 3, it is understood that the heating prior tothe diameter-reducing rolling is preferably set to be not lower than thetemperature Ac₁ from the standpoint of suppressing the thicknessdeviation and wrinkles occurred near the seam. The temperature Ac₁depends on the chemical composition of the steel pipe, etc., and can bedetermined based on experiments. A range of temperature Ac₁ isapproximately not lower than 800° C. However, if the heating temperatureis too high, the crystal grain size would be excessively increased, thusresulting in a problem of, for example, increasing surface roughnessduring the working. For that reason, the heating temperature ispreferably set to be not higher than 900° C.

There is no particular restriction upon the cooling after the heating ofthe steel pipe. Subsequent to the heating, the diameter-reducing rollingmay be performed, for example, after cooling the steel pipe down totemperatures at which ferrite forms the main phase, or by reheating thesteel pipe after cooling it down to the room temperature.

Further, preferably, after the diameter-reducing rolling of the steelpipe, heat treatment of holding the rolled steel pipe in a temperaturerange of from 600° C. to 900° C. for a time of 1 second or longer isperformed.

Since the diameter-reducing rolling is performed at temperatures of notlower than 600° C., the work hardness is low and a sufficient level ofworkability is obtained with additional treatment. Even so, byperforming heat treatment for holding the rolled steel pipe at a certaintemperature for a certain time in succession to the diameter-reducingrolling, the elongation and the r-value are further improved. Thiseffect is developed by holding the rolled steel pipe at temperatures ofnot lower than 600° C. for a time of 1 second or longer. However, if theholding temperature exceeds 900° C., the steel structure would betransformed into the single austenitic phase and the r-value would bereduced because of the randomized aggregation structure. For thatreason, the heat treatment is preferably performed on conditions of theholding temperature in the range of from 600° C. to 900° C. and theholding time of 1 second or longer. Additionally, the heat treatment maybe performed during cooling subsequent to the diameter-reducing rollingor by reheating the rolled steel pipe after the cooling.

EXAMPLE

Seam welded steel pipes were produced by an ordinary method from variouskinds of hot-rolled steel plates having chemical compositions shown inTable 1, and the diameter-reducing rolling was performed on each steelpipe under conditions shown in Table 2. Heating of the steel pipe priorto the diameter-reducing rolling was not held at all or held for a timeof 1 to 600 seconds after reaching the temperature shown in Table 2.Tensile specimens of JIS No. 12-A were sampled from circumferentialpositions 0°, 90°, 180° and 270° of each steel pipe obtained. Afterbonding a strain gauge with a gauge length of 2 mm to each specimen, atensile test was carried out on the specimen by applying a nominalstrain of 6 to 7%. Then, a ratio of a true strain ε_(w) in the widthdirection to a true strain ε_(L) in the longitudinal direction wasmeasured. From a gradient ρ of that ratio, the r-value was calculatedbased on the following formulae:ρ=ε_(L)/ε_(w)r-value=ρ/(−1−ρ)

Further, a thickness deviation η was calculated by measuring a pipe wallthickness ts of a seamed portion and an average pipe wall thickness tbof the remaining portion. That is:thickness deviation η%=(ts−tb)/tb×100%

Moreover, the presence or absence of wrinkles was determined byobserving an image of an area near the seam in a cross-sectionperpendicular to the axis of the steel pipe, the image being enlarged ata magnification of 50 times.

Those results are listed in Table 3 along with the tensile strength (TS)and the elongation (E1).

The r-value is 1.2 or above at any positions in the circumferentialdirection in our Examples, whereas the r-value is below 1.2 inComparative Examples. Also, in the specimens heated to temperatures ofnot lower than Ac₁, the thickness deviation is smaller and wrinkles arenot caused.

INDUSTRIAL APPLICABILITY

A high-workability steel pipe can be provided which has a high r-valueover an entire area in the circumferential direction, including a seamedportion, and also has a good shape. Limits in bending and expanding workof the steel pipe are noticeably improved, whereby omission of steps dueto the integral forming and a reduction in weight can be achieved.Further, seam welded steel pipes having a high r-value can also beproduced using, as base materials, hot-rolled steel, high tensilestrength steel such as dual phase steel, and low, medium and high carbonsteel, which have a difficulty in achieving a high r-value with aconventional method of producing a steel pipe by simply seam-welding asteel plate. As a result, we are able to remarkably enlarge theapplicable range of bending of steel pipes and hence greatly contributesto development of the industry.

TABLE 1 Chemical Composition (&) Ac₁ Ac₃ Steel C Si Mn P S Al N Cr Ti NbB Ni Cu (° C.) (° C.) A 0.06 0.1 0.3 0.01 0.005 0.02 0.003 — — — — — —730 840 B 0.1 0.2 0.8 0.01 0.005 0.02 0.003 — — — — — — 730 820 C 0.250.3 0.8 0.01 0.005 0.02 0.003 — — — — — — 750 800 D 0.25 0.3 0.5 0.010.005 0.02 0.003 — — — 0.002 — — 750 800 E 0.4 0.3 1.6 0.01 0.005 0.020.003  0.03 — — — — — 730 780 F 0.08 1.0 1.4 0.01 0.005 0.02 0.003  0.90.01 — — — — 750 840 G 0.15 1.4 1.5 0.01 0.005 0.02 0.003  0.3 — — — — —770 820 H 0.08 0.5 1.2 0.01 0.005 0.02 0.003 — 0.04 — — — — 770 820 I0.08 0.04 1.5 0.01 0.005 0.02 0.003 — 0.04 — — — — 750 800 J 0.08 1.51.8 0.01 0.005 0.02 0.003 — 0.1  — — — — 780 830 K 0.09 0.05 1.8 0.010.005 0.02 0.003 — 0.15 0.05 — — — 750 800 L 0.01 0.2 1.5 0.01 0.0050.02 0.003 11.0 — — — 0.25 0.4 730 800

TABLE 2 Incoming-side Outgoing-side Total Effective TemperatureTemperature Reduction Reduction Heating in Diameter- in Diameter- in inTemperature Reducing Reducing Diameter Diameter* Heat No. Steel (° C.)Rolling (° C.) Rolling (° C.) (%) (%) Treatment Remarks 1 A 800 780 73050 50 — Example 2 A 900 880 830 50 5 — Comparative Example 3 A 630 610560 50 10 — Comparative Example 4 B 800 780 730 50 50 — Example S B 800780 730 50 50 — Example 6 C 800 780 730 50 50 730° C. × Example 5 min. 7D 900** 720 680 50 50 — Example 8 D 850 720 680 50 50 — Example 9 D 800780 730 50 50 — Example 10 D 800 720 680 50 50 — Example 11 D 750 720680 50 50 — Example 12 D 735 720 680 50 50 — Example 13 D 720 720 680 5050 — Example 14 E 800 780 730 50 50 — Example 15 F 800 780 730 0 0 —Comparative Example 16 F 800 780 730 15 15 — Comparative Example 17 F800 780 730 30 30 — Example 18 F 800 780 730 40 40 — Example 19 F 800780 730 50 50 — Example 20 F 800 780 730 60 60 — Example 21 F 800 780730 70 70 — Example 22 F 900 890 850 30 2 — Comparative Example 23 F 850840 780 30 30 — Example 24 F 750 730 680 30 30 — Example 25 F 700 680600 30 30 — Example 26 F 630 610 560 50 10 — Comparative Example 27 G900 780 730 50 50 — Example 28 G 850 780 730 50 50 — Example 29 G 800780 730 30 30 — Example 30 G 800 780 730 40 40 — Example 31 G 800 780730 50 50 — Example 32 H 800 780 730 50 50 — Example 33 I 800 780 730 5050 — Example 34 J 800 780 730 50 50 — Example 35 K 800 780 730 50 50 —Example 36 L 760 740 700 60 60 — Example *effective reduction indiameter: reduction in diameter in temperature range of 600° C. to Ac₃**rolling after cooling and reheating (for other types of steel, rollingimmediately after heating)

TABLE 3 Wrinkles Seam ∘ not 0° (Seam) 90° 180° 270° Thickness occurredTS/ EI* r- TS/ EI* r- TS/ EI* r- TS/ EI* r- Deviation x No Mpa /% valueMpa /% value Mpa /% value Mpa /% value /% occurred Remarks 1 300 55 2.0303 54 2.0 307 54 2.1 301 55 2.1 0.3 ∘ Example 2 300 45 0.8 309 45 0.9307 45 0.8 308 45 0.8 0.3 ∘ Comparative Example 3 450 35 1.0 450 35 1.1459 36 1.0 451 34 1.1 10.0 X Comparative Example 4 350 50 2.0 356 51 2.0356 50 2.0 350 51 2.0 0.5 ∘ Example 5 350 50 2.4 358 51 2.4 351 49 2.5356 49 2.4 0.5 ∘ Example 6 620 25 1.8 624 24 1.8 625 25 1.8 629 25 1.90.3 ∘ Example 7 640 27 1.7 646 27 1.7 641 27 1.7 647 26 1.7 0.5 ∘Example 8 631 25 1.7 651 26 1.6 641 25 1.8 641 25 1.8 1.0 ∘ Example 9620 28 1.8 626 29 1.8 621 29 1.9 627 28 1.9 0.5 ∘ Example 10 640 24 1.6659 24 1.7 632 24 1.7 636 24 1.7 2.0 ∘ Example 11 644 22 1.6 650 22 1.7635 22 1.7 632 22 1.8 30 ∘ Example 12 653 20 1.6 657 21 1.6 640 21 1.8623 21 1.8 8.0 x Example 13 644 19 1.7 650 19 1.7 637 19 1.9 614 19 1.815.0 x Example 14 650 25 1.8 652 25 1.9 651 25 1.8 651 26 1.9 0.5 ∘Example 15 500 25 0.7 508 26 0.8 503 24 0.8 SOl 25 0.8 0.3 ∘ ComparativeExample 16 590 28 1.0 593 28 1.1 599 29 1.1 595 28 1.0 0.3 ∘ ComparativeExample 17 610 28 1.3 610 28 1.3 618 28 1.3 614 29 1.3 0.9 ∘ Example 18610 29 1.4 619 29 1.4 611 30 1.4 611 28 1.4 0.9 ∘ Example 19 610 30 1.66l7 31 1.7 611 30 1.6 61S 31 1.6 0.9 ∘ Example 20 610 32 2.0 616 31 2.0612 33 2.1 610 31 2.1 0.9 ∘ Example 21 610 35 2.5 615 35 2.6 613 36 2.6618 36 2.6 0.8 ∘ Example 22 590 28 0.8 593 27 0.8 599 28 0.8 593 28 0.90.2 ∘ Comparative Example 23 610 29 1.4 612 30 1.4 614 30 1.5 616 29 1.50.2 ∘ Example 24 610 28 1.3 613 29 1.3 615 28 1.4 612 28 1.4 0.0 ∘Example 25 650 27 1.2 651 26 1.2 650 27 1.2 658 26 1.2 3.0 x Example 26630 22 0.9 680 21 1.0 687 22 1.0 685 23 0.9 15.0 x Comparative Example27 630 30 1.3 638 30 1.3 639 31 1.4 640 31 1.3 0.7 ∘ Example 28 630 331.4 636 33 1.4 630 33 1.5 638 33 1.5 0.5 ∘ Example 29 630 30 1.3 638 301.3 639 31 1.4 640 31 1.3 0.3 ∘ Example 30 630 33 1.4 636 33 1.4 630 331.5 638 33 1.5 0.3 ∘ Example 31 630 35 1.8 637 34 1.9 635 35 1.8 633 341.9 0.4 ∘ Example 32 600 30 1.8 606 30 1.8 609 30 1.9 600 30 1.8 0.5 ∘Example 33 600 30 1.8 604 29 1.8 605 31 1.9 601 29 1.9 0.8 ∘ Example 34820 24 1.6 823 25 1.6 821 25 1.7 825 24 1.7 0.3 ∘ Example 35 820 22 1.6821 22 1.6 823 23 1.7 830 22 1.7 0.8 ∘ Example 36 695 28 1.8 595 28 1.8595 28 1.8 595 28 1.8 0.3 ∘ Example *sheet thickness = 1.6 mm

1. A method of producing a high-workability steel pipe comprising:seam-welding strip steel into a steel pipe; heating the steel pipe totemperatures of more than Ac₃; and immediately or after cooling andreheating said steel pipe, performing diameter-reducing rolling of thesteel pipe in a temperature range of from 700° C. to Ac₃ with areduction in diameter of the steel pipe of not less than 30% such thatthe pipe and a weld in the pipe have an r-value of 1.3 or more.
 2. Themethod of producing a high-workability steel pipe according to claim 1,wherein after the diameter-reducing rolling of said steel pipe, heattreatment of holding the rolled steel pipe in a temperature range offrom 600° C. to 900° C. for a time of 1 second or longer is performedduring cooling subsequent to the diameter-reducing rolling or byreheating the rolled steel pipe after said cooling.
 3. The methodaccording to claim 1, wherein heating the steel pipe is at Ac₃ to 900°C.
 4. The method according. to claim 1, wherein the Ac₃ temperature is840° C.
 5. A method of producing a high-workability steel pipecomprising: seam-welding steel strip into a steel pipe; heating thesteel pipe to temperatures of more than Ac₃; and immediately afterheating the steel pipe, performing diameter-reducing rolling of thesteel pipe in a temperature range of from 700° C. to Ac₃ with areduction in diameter of the steel pipe of not less than 30% such thatthe pipe and a weld in the pipe have an r-value of 1.3 or more.
 6. Themethod according to claim 5, further comprising: after diameter-reducingrolling the steel pipe, performing a heat treatment of holding therolled steel pipe in a temperature range of from 600° C. to 900° C. fora time of one second or longer.
 7. The method of according to claim 5,wherein heating the steel pipe is at Ac₃ to 900° C.
 8. The methodaccording to claim 5, wherein the Ac₃ temperature is 840° C.
 9. A methodof producing a high-workability steel pipe comprising: seam-weldingsteel strip into a steel pipe; heating the steel pipe to temperatures ofmore than Ac₃; cooling the heated steel pipe; reheating the cooled steelpipe; and performing diameter-reducing rolling of the steel pipe in atemperature range of from 700° C. to Ac₃ with a reduction in diameter ofthe steel pipe of not less than 30% such that the pipe and a weld in thepipe have an r-value of 1.3 or more.
 10. The method according to claim9, further comprising: after diameter-reducing rolling the steel pipe,performing a heat treatment of holding the rolled steel pipe in atemperature range of from 600° C. to 900° C. for a time of one second orlonger.
 11. The method according to claim 9, wherein heating the steelpipe is at Ac₃ to 900° C.
 12. The method according to claim 9, whereinthe Ac₃ temperature is 840° C.